Use of cordless power tools has increased dramatically in recent years. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Conventionally, cordless tools have been driven by Permanent Magnet (PM) brushed motors that receive DC power from a battery assembly or converted AC power. The motor associated with a cordless tool has a direct impact on many of the operating characteristics of the tool, such as output torque, time duration of operation between charges and durability of the tool. The torque output relates to the capability of the power tool to operate under greater loads without stalling. The time duration of the power tool operation is strongly affected by the energy efficiency of the motor. Since, during some operating modes cordless tools are powered by battery modules that contain a limited amount of energy, the greater the energy efficiency of the motor, the longer the time duration that the tool can be operated. The durability of a power tool is affected by many factors, including the type of motor that is used to convert electrical power into mechanical power.
Brushed motors such as the PM brushed motors that are generally employed in power tool applications are susceptible to damaged brushes over time. The main mechanical characteristic that separates Permanent Magnet brushless motors from Permanent Magnet brushed motors is the method of commutation. In a PM brushed motor, commutation is achieved mechanically via a commutator and a brush system. Whereas, in a brushless DC motor, commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor includes a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core includes field windings around the rotor. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings. A set of sense magnets coupled to the PMs in the rotor assembly are sensed by a sensor, such as a Hall Effect sensor, to identify the current position of the rotor assembly. Proper timing of the commutation sequence is maintained by monitoring sensors mounted on the rotor shaft or detecting magnetic field peaks or nulls associated with the PM.
While some power tool applications such as hammer drills require high power motors, some power tools such as certain fastening tools require less power output from the motor. For example, a finish nailer requires less energy as compared to other nailing applications such as framing, fencing or concrete. This is because finish nails are relatively thin (16 Ga or 18 Ga) and require less energy to be fired, whereas, by comparison, concrete nails are thicker and require joining steel beams with concrete, and therefore require more energy.
The main user critical-to-quality requirement for a finish nailer is small size and light weight. Thus, in such power tool applications it is important to assemble and package motor components, including the motor and the control module, in a compact and efficient manner. What is needed is a control module that is packaged and assembled compactly and efficiently.
Furthermore, various applications have different levels of thermal management requirement associated with the power requirement of the tool, even though the control methods for these applications are functionally similar. What is needed is a control module design that can be easily configured to meet various thermal requirements of the power tool with minimal level of modification.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.